The “can’t block” effect applies to each of the targeted creatures—and to only those creatures—even if Wrap in Flames deals no damage to one or more of them (due to a prevention effect, for example) or if Wrap in Flames deals damage to a different creature (due to a redirection effect).

If any of the targeted creatures is an illegal target by the time Wrap in Flames resolves, it won’t be dealt damage and will be able to block. The other targeted creatures will still be affected.